Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices (“IMDs”). These capacitors are required to have a high energy density, since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 475 volt electrolytic capacitors in series to achieve a voltage of 700 to 950 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is typically used for the anode foils, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used.
A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent.
Electrolytic capacitors are typically formed into flat or cylindrical shapes. For a flat construction, the individual cathode and anode foils or plates are stacked in an interleaved manner with separators interposed there between. For a cylindrical construction, the stacked plates are then rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. In both the flat and cylindrical constructions, connections to the anode and the cathode are made via tabs.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Typically, aluminum electrolytic capacitors are designed to increase capacitance by increasing the surface area per projected area of anodes. This requires a higher effective surface area and capacitance from corresponding cathodes to realize all of the anodes' capacitance.
The present disclosure will be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left most digit(s) of a reference number identifies the drawing in which the reference number first appears.